We have developed an animal model for studying the effects of excessive prolactin release (hyperprolactinemia) on male reproductive and sexual functions. Transplantation of four pituitary glands under the kidney capsules of adult male rats produces a chronic significant elevation of circulating prolactin (PRL) levels, reduction in gonadotropin (LH & FSH) release, increase in testicular LH binding and deficits of copulatory behavior. We have already shown that effects of grafts on sexual behavior are due to PRL secreted by the grafts and that this effect is not mediated by stimulation of adrenal activity. We now propose to determine if deficits of copulatory behavior in hyperprolactinemic male rats can be corrected by administration of LHRH, testosterone or estradiol and whether pituitary grafts suppress mating behavior in gonadectomized animals treated with testosterone. In a series of studies in vivo and in vitro, we will compare testicular responsiveness to gonadotropins in hyperprolactinemic and control rats. Finally, we will study behavioral and hormonal correlates of very severe hyperprolactinemia induced by transplantation of PRL-producing tumors. The results should indicate by what mechanisms excessive PRL release can interfere with libido, potency and hormone production by the testes.